Residual stresses in welded joints of work hardening steels with high manganese content

对于车身结构中高强度结构部件的开发，具有延展性和强度最佳组合的钢材的应用特别令人感兴趣。具有高锰含量的钢是非常令人感兴趣的，因为最终的机械性能是通过变形诱导马氏体形成（TRIP效应）或通过孪晶界的形成（TTRIP效应）来实现的。在变形状态下，这些钢联合收割机结合了高强度和高塑性变形储备。对于由这些材料制成的汽车部件领域中的焊接结构的设计来说，重要的是焊接接头必须表现出非常高的强度，即使对于疲劳载荷条件。在这方面，目前还不清楚在何种程度上焊接引起的残余应力将影响强度和疲劳强度行为，以及在何种程度上第二和第三类的微观残余应力的贡献。此外，还不知道微观残余应力发展的各种原因与宏观残余应力的稳定性之间是否存在关系。通过回答这些问题，研究项目将作出重大贡献，以获得更好地了解加工特性和由此产生的强度强加工硬化奥氏体钢，并提供了一个改进的基础上，定量考虑不同的残余应力份额的组件设计。该项目旨在理解和描述由成形过程引起的宏观残余应力与焊接后状态之间的相互关系。这将通过系统的实验和通过过程链变形-焊接的有限元模拟来实现，在这方面，对由高锰含量的钢制成的焊接样品，即X40 MnCrVAl 19 2.5（TMn）和X10 Mn 7（TRIP 700），用于分析局部相含量，相特定的织构演变和发展和稳定的相特定的残余应力在所有尺度的计划。对工艺参数与所产生的微观和宏观残余应力之间的关系的理解将为通过优化热输入和冷却过程以及焊接工艺本身来优化由这些合金制成的部件的准静态和循环强度提供基础。对于有限元模拟，将通过成形和焊接过程中塑性变形的局部计算以及与实验结果的耦合来提供与微观残余应力的相关性。

For the development of high strength structural components in body construction the application of steels with an optimal combination between ductility and strength is of particular interest. Steels with high manganese contents are highly interesting as the final mechanical properties are achieved through deformation induced martensite formation (TRIP effect) or through formation of twin boundaries (TWIP effect). In the deformed state these steels combine high strength with high plastic deformation reserves. Important for the design of weld constructions from instance in the field of automotive components made from these materials is that the welded joints must exhibit very high strength even for fatigue load conditions. In this regard it remains unclear to what extend welding induced residual stresses will affect the strength and fatigue strength behavior and to what extend the micro residual stresses of 2nd and 3rd kind contribute. Furthermore, it is unknown if there is a relationship between the various causes for the development of micro residual stresses and the resulting stability of macro residual stresses. By answering these questions the research project will make a significant contribution to gain a better understanding of the processing characteristics and the resulting strength of strongly work hardening austenitic steels and provides an improved basis for the quantitative consideration of different residual stress shares for components design. The project aims at the understanding and description of the interrelations between the macro residual stresses induced by the forming process and the state after welding. This will be realized by systematic experiments and through finite element simulations of the process chain deformation-welding.In this regard, extensive systematic X-ray diffraction analyses for welded samples made of steel with high manganese content, i.e. X40MnCrVAl19 2.5 (TWIP) and X10Mn7 (TRIP 700), for analyzing local phase contents, phase-specific texture evolutions and the development and stability of phase-specific residual stresses at all scales are planned. The comprehension of the relationship between process parameters and resulting micro and macro residual stresses will provide the basis for optimizing quasi-static and cyclic strength of components made from these alloys through optimizing the heat input and the cooling course and hence of the welding process itself. For the FE simulations the correlation with micro residual stresses will be provided through local calculation of plastic deformations during forming and welding and coupling with experimental results.